1. Correct Answer: The correct answer is c: 13.
 2. Explanation:
         ○ To find the missing number 'X' in the series, we need to identify the pattern or rule governing the sequence.
         ○ Let's examine the differences between consecutive numbers in the series:
           ○ The difference between 21 and 31 is 10.
           ○ The difference between 31 and 43 is 12.
         ○ Notice that the differences are increasing by 2 each time (10, 12, ...).
         ○ Let's apply this pattern to find the missing number 'X':
           ○ The difference between 7 and 'X' should be 8 (since the difference between 21 and 31 is 10, and the difference between 31 and 43 is 12, the difference before 10 should be 8).
           ○ Therefore, \( X = 7 + 8 = 15 \).
         ○ However, upon re-evaluation, the correct pattern should be:
           ○ The difference between 7 and 'X' should be 6 (since the differences are increasing by 2, starting from 6).
           ○ Therefore, \( X = 7 + 6 = 13 \).
         ○ Thus, the missing number 'X' is 13.